Method for measuring and calibrating measurements using optical fiber distributed sensor

ABSTRACT

Methods for calibrating and making measurements using fiber optic sensors are disclosed using backscattered wavelengths and independent sensors. The disclosure sets outs methods applicable with fiber optic sensors either in a deployed in a loop and in a linear configuration and useful for measurements including temperature.

BACKGROUND OF THE INVENTION

[0001] The invention relates to fiber optic distributed sensors andmethods of measuring parameters and calibrating parameter measurementsmade using optical fiber distributed sensors. In particular, methods ofmeasuring temperature and calibrating temperature measurements madeusing fiber optic distributed temperature sensors are disclosed.

DESCRIPTION OF RELATED ART

[0002] Optical fibers typically include a core, a concentric claddingsurrounding the core, a concentric protective jacket or buffersurrounding the cladding. Generally the core is made of transparentglass or plastic possessing a certain index of refraction and thecladding is made of transparent glass or plastic possessing a differentindex of refraction. The relative refractive indices of the core and thecladding largely determine the function and performance of the opticalfiber. As a beam of light is introduced into the optical fiber, thevelocity and direction of the light changes at the interface of mediawith different refraction indices. The angles of reflection andrefraction can be predicted using Snell's law if the refractive indicesof both media are known. It is known to alter the media with theirrespective refraction indices to provide optical fiber with certainlight propagating characteristics. Typically, for minimal power loss, itis desirable for the light to propagate mainly through the core of theoptical fiber. In addition to refraction indices, other factors thataffect the propagation of the light through the fiber optic core includethe dimensions of the core and the cladding, the wavelength of thelight, the magnetic field vectors of the light and the electrical fieldvectors of the light, the configuration of the optical fiber, thepresence of imperfections, and environmental effects such as bends,twists, creases or folds.

[0003] One advantage of optical fiber is the ability to determineinformation concerning a parameter of interest relating to environmentaleffects along the length of a fiber. Measurements are made byintroducing optical energy into an optical fiber and receivingbackscattered light returned from various distances along the opticalfiber. In order to relate the characteristics of backscattered light tothe parameter of interest at a particular distance, it is known to useoptical time domain reflectometry (OTDR) to determine the distance fromwhich the light is returned along the fiber is required. Such methodsare known and described in U.S. Pat. No. 4,823,166 to Hartog et al. andU.S. Pat. No. 5,592,282 to Hartog, both of which are incorporated hereinin the entirety by reference. In OTDR, a pulse of optical energy isintroduced to the optical fiber and the backscattered optical energyreturning from the fiber is observed as a function of time, which isproportional to distance along the fiber from which the backscatteredlight is received. This backscattered light includes the Rayleighspectrum, the Brillouin spectrum, and the Raman spectrum. The Ramanspectrum is the most temperature sensitive with the intensity of thespectrum varying with temperature, although all three types ofbackscattered light contain temperature information.

[0004] Fiber optic (FO) sensors employ the fact that environmentaleffects can alter the amplitude, phase, frequency, spectral content, orpolarization of light propagated through an optical fiber. Fiber opticssensors can be classified as intrinsic or extrinsic. Intrinsic sensorsmeasure ambient environmental effects by relying on the properties ofthe optical fiber only while extrinsic sensors are coupled to anotherdevice to translate environmental effects into changes in the propertiesof the light in the fiber optic. Intrinsic fiber optic distributedtemperature sensors (DTS) are known. One such device is disclosed inU.S. Pat. No. 5,825,804 to Sai, incorporated herein in its entirety byreference. Such sensors may be multimode fiber (MMF) or single modefiber (SMF). Single mode optical fibers have a relatively small diameterand support only one spatial mode of propagation. Multimode fibers havea core with a relatively large diameter and permit non-axial rays ormodes to propagate through the core.

[0005] Typically the Raman spectrum is used to measure temperature, thetemperature distribution is calculated based on the ratio of between theStokes component and the anti-Stokes component of the Raman spectrum ofthe backscattered light as follows: $\begin{matrix}{\frac{I_{as}}{I_{s}} = {\beta \quad {\exp \lbrack \frac{- {hcv}}{kT} \rbrack}}} & (1)\end{matrix}$

[0006] where β is a coefficient, h is Plank's constant, υ is Ramanshifted wavelength number, k is Bolzmann constant, T is absolutetemperature, I_(as) is the anti-Stokes component and I_(s) is the Stokescomponent. The Stokes/anti-Stokes ratio I_(s)/I_(as) SAR. Using Equation1, the temperature at the position along the optical fiber from whichthe backscattering occurred can be determined.

[0007] To measure temperature along a fiber optic (FO) distributedsensor, optical energy is introduced into the fiber and backscatteredlight is excited. The backscattered signal (light) contains informationrelating to the point along the fiber from which the backscatteringoccurred. This light is sensed and processed as a time-sequence signal.A one-dimensional temperature distribution along the optical fiber isthus measured. Within the backscattered light, typically the Ramanspectrum is transferred by an optical directional coupler to a measuringapparatus, whereby the Stokes light and the anti-Stokes light in theRaman backscattered light are separated by a filter, detected, andconverted to electrical signals in proportion to their associatedamplitudes by respective photo-electric converters. It is known tocalculate temperature distribution based on the ratio between thesecomponents of backscattered light, or alternatively based on measurementof only one component of the Raman spectrum of backscattered light.

[0008] In optical fiber, there are losses that can affect backscatteredStokes and anti-Stokes wavelengths differently. For example, the opticalenergy introduced into the optical fiber naturally undergoes attenuationduring transmission through the fiber. Also there may losses owing toenvironmental stresses like bends or connections. These losses subtractdifferently from the measured backscattered Stokes and anti-Stokesintensities. These differences in fiber attenuation between Stokes andanti-Stokes wavelengths must be addressed to avoid error in the measuredparameter along the FO distributed sensor.

[0009] Parameter measurements obtained using a FO distributed sensorcomprise the true parameter measurement and a measurement error causedby deleterious influences on the fiber optic distributed sensor. By wayof example but not limitation, such deleterious influences can includeenergy losses due to splices or bends, strains in the fiber, changes inattenuation resulting from aging or environmental conditions, drift inmeasurements over time, hydrogen ingression, or environmentalconditions. Such error is cumulative with distance along a fiber. Whilecertain measurement errors can be predicted based on manufacturer ormaterial calibration information, baseline testing, or tracking of knownelements such as splice location, the occurrence and effect of otherdeleterious influences and the measurement error they introduce isdifficult to assess. It is known to deploy an optical fiber in aborehole to obtain distributed measurements of borehole parameters andit can be appreciated that accounting for these deleterious influencesand their associated measurement error is particularly difficult whenthe fiber optic distributed sensor is deployed in a borehole. A needexists for a method of calibrating FO distributed sensors and aparticular need exists for a method of calibrating optical fibersdeployed in a borehole for use in distributed temperature measurements.

[0010] One method for correction is presented in U.S. Pat. No. 5,102,232issued to Tanabe et al. However this method requires maintaining anoptical fiber temperature reference point at a known temperature.Maintaining such a reference point may not be feasible. For example, indownhole application where an optical fiber is disposed in a borehole,it may not be possible to maintain a reference point at a knowntemperature.

[0011] Robust methods for accurately determining parameters using a FOdistributed sensor measurements in a borehole are needed. The accuracyof parameter measurements can be limited by the algorithm or methodologyused to account for variations in the measurements and such limitationsin methodology can exist regardless of whether an optical fiber isdeployed in a borehole in a linear or loop configuration. Methods ofcalibrating parameter measurements obtained using a FO distributedsensor are useful. Methods of measuring a parameter obtained using a FOdistributed sensor that include calibration of the parameter measurementare also useful. A particular need exists for methods of calibratingtemperature measurements obtained using a fiber optic distributedtemperature sensor (FO-DTS) and methods of measuring temperature using aFO-DTS that include calibration of the measurements.

SUMMARY OF THE INVENTION

[0012] The present invention comprises methods of calibrating aparameter measurement obtained using a fiber optic distributed sensorand methods of making a parameter measurement that include calibration.

[0013] One embodiment of the present invention is a method of measuringa parameter using a FO distributed sensor comprising the steps of:measuring a parameter of interest along a FO distributed sensor;measuring said parameter of interest using an independent sensor locatedalong the length of the FO distributed sensor; determining thedifference in the parameter measurement ΔT₁ between the parametermeasurement made by the independent sensor and the parameter measurementmade by the distributed sensor at the location of the independentsensor; and adjusting the parameter measurements determined by the FOdistributed sensor by ΔT₁. In an embodiment, the FO distributed sensoris deployed in a borehole. In a particular embodiment, the independentsensor is placed at a location within an area of particular interest ina borehole. In one embodiment, the parameter of interest is temperature,the FO distributed sensor is a fiber optic distributed temperaturesensor (FO-DTS), and the ΔT₁ is the difference between the temperaturemeasured by an independent temperature sensor and the temperaturemeasured by a FO-DTS.

[0014] Another embodiment of the present invention is a method ofcalibrating a fiber optic distributed sensor, comprising the steps of:measuring a parameter of interest along a FO distributed sensor;measuring said parameter using an independent sensor located along thelength of the FO distributed sensor; determining the difference ΔT₁between the parameter measurement made by the independent sensor and theparameter measurement made by the distributed sensor at the location ofthe independent sensor; and using ΔT₁ to adjust the parametermeasurements determined by the optical fiber distributed sensor. In afurther embodiment, the FO distributed sensor is deployed in a borehole.In yet a further embodiment, the independent sensor is placed at alocation within an area of particular interest in a borehole. In oneparticular embodiment, the parameter of interest is temperature, the FOdistributed sensor is a FO-DTS, and the ΔT₁ is the difference betweenthe temperature measured by an independent temperature sensor and thetemperature measured by a FO-DTS.

[0015] One embodiment of the present invention is a method forcalibrating a FO distributed sensor comprising providing optical energyat the Stokes wavelength to FO distributed sensor; receivingbackscattered signal at the Stokes wavelength and measuring; providingoptical energy at the anti-Stokes wavelength to the FO distributedsensor; receiving backscattered signal at the anti-Stokes wavelength andmeasuring its intensity; calculating the attenuation ratio between thebackscattered Stokes and anti-Stokes wavelengths at points along the FOdistributed sensor using OTDR; repeating these steps at a differenttime; calculating the change in the attenuation ratio with time of theStokes and anti-Stokes wavelengths; and applying said calculated changewith time in attenuation ratio of the Stokes and anti-Stokes wavelengthsto parameter measurements made using a FO distributed sensor. Oneparticular embodiment of this method of calibration is where theparameter is temperature and the FO distributed sensor is a FO-DTS.

[0016] One embodiment of the present invention is a method of measuringtemperature using a FO-DTS comprising providing optical energy at to theStokes wavelength to the FO-DTS; receiving backscattered signal at theStokes wavelength and measuring its intensity; providing optical energyat the anti-Stokes wavelength to the FO-DTS; receiving backscatteredsignal at the anti-Stokes wavelength and measuring its intensity;calculating the attenuation ratio between the backscattered signals atthe Stokes and anti-Stokes wavelengths at points along the optical fiberusing OTDR; repeating these steps at a different time; calculating thechange in the attenuation ratio with time of the Stokes and anti-Stokeswavelengths; and applying the calculated change in attenuation ratio ofthe Stokes and anti-Stokes wavelengths to temperature measurements alongthe FO-DTS.

[0017] Another embodiment of the present invention is a method tocalculate a corrected SAR comprising providing optical energy at theStokes wavelength to an optical fiber; receiving backscattered signal atthe Stokes wavelength and measuring its intensity; providing opticalenergy at the anti-Stokes wavelength to the optical fiber; receivingbackscattered signal at the anti-Stokes wavelength and measuring itsintensity; calculating the attenuation ratio between the backscatteredStokes and anti-Stokes wavelengths at points along the optical fiberusing OTDR; repeating these steps at a different time; calculating thechange in the attenuation ratio with time of the Stokes and anti-Stokeswavelengths; and multiplying a measured SAR by a correction factor,wherein the correction factor comprises the calculated change with timein attenuation ratio of the Stokes and anti-Stokes wavelengths.

[0018] One embodiment of the present invention is a method ofdetermining temperature along a FO-DTS comprising the steps of:measuring the temperature along an FO-DTS; measuring the temperature atone or more locations along the FO-DTS using at least one independenttemperature sensors, determining the difference ΔT_(1i) between thetemperature measured using by each of the least one independenttemperature sensor and the temperature measuring along the FO-DTS at therespective locations of the at least one independent temperature sensor,and adjusting the temperatures measured by the FO-DTS by ΔT_(1i),wherein i is the number of independent temperature sensors provided.

[0019] Another embodiment of the present invention is a method ofcalibrating a FO-DTS, comprising the steps of: measuring the temperaturealong an FO-DTS; measuring the temperature at one or more locationsalong the FO-DTS using at least one independent temperature sensors,determining the difference ΔT_(1i) between the temperature measuredusing by each of the least one independent temperature sensor and thetemperature measuring along the FO-DTS at the respective locations ofthe at least one independent temperature sensor, and using ΔT_(1i) tocalibrate temperature determined by the FO-DTS, wherein i is the numberof independent temperature sensors provided.

[0020] The present invention includes a method of determiningtemperature comprising the steps of measuring the temperature along anFO-DTS; measuring the temperature at one or more locations along theFO-DTS using at least one fiber Bragg grating (FBG) in the FO-DTS,determining the difference ΔT_(2j) between the temperature measuredusing by each of the least one FBG and the temperature measured alongthe FO-DTS at each respective FBG location, and using ΔT_(2j) to adjustthe temperature measured by the FO-DTS, wherein j is the number of FBGtemperature sensors provided.

[0021] Another embodiment of the present invention is a method ofcalibrating a FO-DTS, comprising the steps of: measuring the temperaturealong an FO-DTS; using at least one FBG in the FO-DTS, measuring thetemperature at one or more locations along the FO-DTS using at least oneFBG, determining the difference ΔT_(2j) between the temperature measuredusing by each of the least one FBG and the temperature measuring alongthe FO-DTS at the respective locations of the at least one FBG, andusing ΔT_(2j), to adjust temperatures measured using the FO-DTS, whereinj is the number of FBG temperature sensors provided.

[0022] An embodiment of the present invention is a method of determiningtemperature along a FO-DTS, comprising the steps of: measuring thetemperature along FO-DTS; measuring the temperature at one or morelocations along the FO-DTS using at least one independent temperaturesensors, determining the difference ΔT_(1i) between the temperaturemeasured by each of the least one independent temperature sensor and thetemperature measured along the FO-DTS at the respective locations of theat least one independent temperature sensor, proving at least one FBG inthe FO-DTS, measuring the temperature at one or more locations along theFO-DTS using at least one FBG, determining the difference ΔT_(2j)between the temperature measured using by each of the least one FBG andthe temperature measuring along the FO-DTS at the respective locationsof the at least one FBG, and adjusting the temperatures measured by theFO-DTS based on ΔT_(1i) and ΔT_(2j), wherein i is the number ofindependent temperature sensors and j is the number of FBG temperaturesensors provided.

[0023] An embodiment of the present invention comprises a method ofcalibrating an optical fiber distributed sensor, comprising the stepsof: measuring a parameter along a distributed sensor, measuring saidparameter using at least one independent sensor; providing at least oneFBG in the optical fiber distributed sensor and measuring said parameterusing the at least one FBG; determining the difference ΔT_(1i) betweenthe parameter measured using by each of the least one independent sensorand the parameter measuring along the FO distributed sensor at thelocation of the at least one independent sensor; determining thedifference ΔT_(2j) between the parameter measured using by each of theleast one FBG and the parameter measured along the FO distributed sensorat the location of the at least one FBG, and using ΔT_(1i) and usingΔT_(2j) to calibrate the parameter measurement as determined by the FOdistributed sensor, wherein i is the number of independent temperaturesensors and j is the number of FBG provided.

[0024] Another embodiment of the present invention is a method ofmeasuring temperature along a FO-DTS, comprising providing opticalenergy into one end of the FO-DTS and transmitting in the forwarddirection; measuring the optical signal at locations i along the opticalfiber distributed temperature sensor; determining the incremental lossvariation in the forward direction; providing optical energy into theopposite end of the FO-DTS and transmitting in the reverse direction;measuring the optical signal at locations i along the FO-DTS;calculating the incremental loss variation at location i in the forwardand reverse directions; and adjusting the temperatures measured by theFO-DTS by the incremental loss variation at location i to thetemperature measured by the FO-DTS at location i.

[0025] Yet another embodiment of the present invention is a method ofcalibrating a FO distributed sensor, comprising providing energy intoone end of an optical fiber and transmitting in the forward direction;measuring the optical signal at locations i along the FO distributedsensor, determining the incremental loss variation of a parameter ofinterest in the forward direction; providing optical energy into theopposite end of a FO distributed sensor and transmitting in the reversedirection; measuring the optical signal at locations i along the opticalfiber distributed sensor, calculating the incremental loss variation ofa parameter of interest in the reverse direction; and applying theincremental loss variation at location i to the measurements of saidparameter of interest made by the FO distributed sensor at location i.

[0026] In further embodiments, the incremental loss variation isdetermined at the Stokes, anti-Stokes, or Stokes and anti-Stokeswavelengths, or with respect to the Stokes/anti-Stokes ratio.

[0027] Particular further embodiments of the above embodiments compriseproviding the fiber optic distributed sensor in a borehole, wherein theparameter measured by the sensor is a borehole parameter, such astemperature, pressure, or fluid composition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is one embodiment of the present invention wherein lightpulses at Stokes and anti-Stokes wavelengths are used.

[0029]FIG. 2 is an embodiment of the present invention wherein a FBG isprovided in a FO-DTS.

[0030]FIG. 3 shows normalized spectrum obtained using a FBG in singlemode optical fiber versus normalized spectrum obtained using a FBG in amultimode optical fiber.

[0031]FIG. 4 shows the spectra reflected in an example case by the FBGusing single mode optical fiber and multimode optical fiber.

[0032]FIG. 5 shows one possible configuration for a FO-DTS system withdownhole calibration utilizing FBG.

[0033]FIGS. 6, 7, and 8 contain data showing temperature and errorversus distance for an example deployment of a FO-DTS in a borehole.

[0034]FIG. 9 shows example temperature data collected in both theforward and reverse direction and data corrected for incremental lossvariations between Stokes and anti-Stokes wavelengths.

DETAILED DESCRIPTION

[0035] A concern in using FO distributed sensors is the possible loss ofcalibration of the FO distributed sensor after deployment due to thechange in the fiber characteristics, such as non-uniform attenuation ofthe fiber along the spectrum caused by micro-bending losses or hydrogeningression. This concern is particularly heightened when consideringlong-term deployment of a FO distributed sensor in a boreholeenvironment as changes in the FO cannot be easily monitored, hostileconditions such as high temperature and pressures occur in boreholeenvironments, and the expense of replacing a FO can be significant. Thisconcern can be minimized by methods of providing at least onecalibrating sensor or FBG along the FO distributed sensor, providingmeasurements from each end of a loop configuration FO distributedsensor, and combinations thereof.

[0036] Referring now to FIG. 1, a first embodiment according to thepresent invention is shown. While described in terms of a FO-DTS, it canbe appreciated that the present method is applicable for other types ofFO distributed sensors. The fiber optic distributed temperature sensor20 is placed so as to pass through each area where temperature is to bemeasured. An optical energy source introduces optical energy into theFO-DTS and backscattered signal 22 is generated by the return of theoptical energy along optical fiber. From the backscattered signal, theresponses at the anti-Stokes wavelength and the Stokes wavelength aredetermined and temperature distribution along the FO-DTS is determinedusing Equation 1.

[0037] A DTS optical electronics module 10 is shown connected to aFO-DTS 20. A pulsed light source capable of outputting light at Stokeswavelength 12 is connected to the DTS optical electronic module 10 andFO-DTS 20. Similarly, a pulsed light source capable of outputting lightat the anti-Stokes wavelength 14 is connected to DTS optical electronicsmodule 10 and fiber optic distributed temperature sensor 20. While thepresent method is described in terms of temperature measurements and useof backscattered light at Stokes and anti-Stokes wavelengths, it can beappreciated that the present invention is also applicable to otherdistributed parameter measurements and other wavelength spectrums.

[0038] A pulse of light is provided from the Stokes wavelength lightsource 12 and the intensity of the backscattered light 22 at the Stokeswavelength is measured by the DTS optical electronics module 10. A pulseof light is provided from the anti-Stokes wavelength light source 14 andthe intensity of the backscattered light 22 at the anti-Stokeswavelength is measured by the DTS optical electronics module 10. Theorder in which Stokes and anti-Stokes wavelengths are provided isunimportant. Using OTDR techniques, the DTS optical electronics module10 calculates the attenuation ratio of Stokes versus anti-Stokeswavelengths at various points along the FO-DTS 20.

[0039] This attenuation ratio is stored in memory 16. The process isrepeated again at a known later time and the attenuation ratio from thesecond event is recorded and stored in memory 16. The attenuation ratiofrom the second event is compared to the attenuation ratio from thefirst event using processor 18 and the change of the attenuation ratiowith respect to time is determined.

[0040] Assuming the position of the FO-DTS remains stationary, such aswhen placed in a borehole for monitoring of downhole parameters, thischange of the attenuation ratio with respect to time can be used tocalibrate later measurements made using the FO-DTS. By multiplying thechange in attenuation ratio with respect to time by the period of timethat has passed since the initial measurements, a correction factor ΔARcan be determined.

[0041] Using this correction factor, a corrected SAR can be calculatedas

SAR _(corrected) =ΔAR(SAR _(measured))  (2)

[0042] where SAR_(measured) is the SAR measured at the later time. Thecorrected SAR can be used in Equation 1 to determine more accurately thetemperature measured at later times by an in situ FO-DTS.

[0043] In another embodiment, a tunable light source provides light atStokes and anti-Stokes wavelengths. In this embodiment, a single tunablelight source is used to provide light at Stokes and anti-Stokeswavelengths rather than separate Stokes source 12 and anti-Stokes source14. One such tunable pulsed light source is a pulsed tunable laser.

[0044] Another embodiment of the present invention comprises providing aFO distributed sensor and further providing at least one gauge capableof providing a calibrating measurement along the length of the DTS. Thiscalibrating measurement can be performed by providing at least one fiberBragg grating embedded in the FO distributed sensor or at least oneindependent sensor or combinations thereof. It is known to embed FBG inan optical fiber to sense parameters such as temperature or strain ofthe structure. Such a fiber optic grating system is described in U.S.Pat. No. 5,380,995 to Udd and Clark, incorporated herein in its entiretyby reference. It is also known that the reflection wavelength of thegrating changes with temperature due to the change in refractive indexand grating spacing over temperature.

[0045] Distributed temperature sensors systems can use MMF or SMF; morecommonly MMF is used. Single mode fibers have a core with a relativelysmall diameter and MMF have a relatively large core diameter. Oneembodiment of the present invention is a method of determiningtemperature measurements along a FO-DTS, comprising use of at least oneFBG. A single and easily discernable temperature peak will be indicatedby the FBG if the fiber optic distributed temperature sensor is SMF. Ifthe fiber optic distributed temperature sensor is MMF, multiple peakswill be indicated by the FBG sensor, yielding a less discernabletemperature reading. The present invention contemplates use of a SMF, aMMF, or a combination thereof.

[0046] As shown in FIG. 2, one particular embodiment is to provide aFO-DTS comprising a segment of SMF 34 and a segment of MMF 36, whereinat least one FBG is placed in the SMF 34. This configuration combinesthe advantages of using MMF and SMF. Referring to FIG. 2, an integrationsystem 28 comprising a light source 30 and a spectral analyzer 32 areconnected to a SMF 34. The light source 30 may be, e.g., a LED, atunable laser, or a laser diode while the spectral analyzer 32 may be,e.g., a Fabry-Perot filter, an acouto-optical filter or an opticalspectral analyzer (OSA). A MMF 36 is connected to the SMF 34. Methods ofmaking connections 38 between the MMF and SSF such as splicing or usingconnectors are known. At least one FBG 40 is placed in the SMF 34.

[0047] Two tests were performed to compare the performance of a SMF withthe performance of a SMF with MMF added between the interrogation system28 and the FBG sensor 40. FIG. 3 shows the normalized spectrum reflectedin one test where optical energy was input into the cable and a spectrummeasured using a FBG 40 for a first case 42 comprising a SMF with FBGand a second case 44 comprising a SMF with MMF inserted between the SMFand the FBG. FIG. 3 indicates there was no remarkable degradation of onthe shape of the spectrum or the change in center wavelength.

[0048] In another test, optical energy was input into the cable and aspectrum measured. FIG. 4 shows the spectrum reflected by the FBG in thetwo cases. In a first case 50, FBG 40 was provided in the SMF 34, whichconnected directly to an integration system 28 comprising a light source30 and an OSA 32 and a spectrum was measured. In a second case 52, FBG40 was provided in the SMF 34, which connected to MMF 36 that wasconnected to an integration system 28 comprising a light source 30 andan OSA 32 and a spectrum measured. For these cases, the difference inpower for the SMF spectrum 50 and the SMF with MMF spectrum 52 was shownto be about 10 dB at the FBG center wavelength. Connection loss wasestimated to be approximately 2.5 dB.

[0049] One possible configuration for a DTS system with downholecalibration utilizing FBG is shown in FIG. 5. The distributedtemperature sensor system comprises DTS optical electronics module 60,an OSA 62, and a FO-DTS 64 which can be either MMF or SMF. If the FOdistributed sensor is MMF, a length of SMF 66 may be provided at the endof the MMF cable. Preferably at least one FBG 68 is provided. If a SMFlength 66 is provided in conjunction with MMF, preferably the FBG 68 isprovided on the SMF length. It should be appreciated however that theinvention encompasses providing the at least one FBG 68 on the MMFlength as well as the SMF length and encompasses providing the at leastone FBG 68 on a MMF without associated use of SMF. Each at least one FBGprovides a measurement of temperature at its particular location.

[0050] An independent temperature sensor 72 is used to measure thetemperature at a particular location. This independent temperaturesensor may be a fiber optic temperature sensor or another type oftemperature sensor such as an electrical quartz sensor or a sensorcomprising a crystal quartz gauge. In particular situations where ahighly precise or highly accurate temperature measurement is desired,gauges of the type capable of providing a more precise and accuratetemperature measurement than a DTS can be provided. The temperaturereading of the independent temperature sensor 72 and the temperaturereading of the optical fiber distributed temperature sensor 64 at thelocation of the independent temperature sensor 72 can be compared andthe difference calculated as ΔT₁. This difference can be used tocalibrate the FO-DTS 64 along the length of the FO-DTS 64 by applyingΔT₁ to the temperature readings obtained by the distributed temperaturesensor.

[0051] By providing at least one FBG 68, a temperature correction can bedetermined at the location of the at least one FBG by calculating thedifference ΔT₂ of the FBG temperature measurement and the temperaturemeasurement of the FO-DTS at that location. Another method is tocalibrate the FO-DTS by applying the average of ΔT₁ and ΔT₂ along thelength between the at least two sensors. Such an average may be anarithmetic average or a weighted average based on relative location ofthe temperature reading along the DTS. Alternatively ΔT₁ and ΔT₂ may beapplied spatially along the DTS or either ΔT₁ or ΔT₂ may be applied atvarious locations along the DTS. In particular ΔT₁ and ΔT₂ may beapplied between the locations of the independent sensors or the FBG orat known locations of physical features such as splices, connection orbends.

[0052] While the example shown in FIG. 5 comprises a FBG and twoindependent sensors, the present invention also contemplates use ofmultiple FBG and multiple independent sensors along a FO-DTS. In such acase, ΔT_(1i) represent the difference in the temperature measurementsbetween ith independent temperature sensor and the FO-DTS at thelocation of the ith independent sensor, and ΔT_(2j) represent thedifference in the temperature measurements between the jth FBG andFO-DTS at the location of the jth FBG. Calibration of the FO-DTS can bemade by applying an arithmetic or weighted average of ΔT_(1i) andΔT_(2j); applying a function of ΔT_(1i) and ΔT_(2j); or spatiallyapplying ΔT_(1i) and ΔT_(2j) based on known characteristics of theFO-DTS such as the locations of splices in the optical fiber.

[0053] In an example of one embodiment of the present invention, a fiberoptic DTS and three independent temperature gauges were deployed in ahorizontal well, the well having both a vertical section and ahorizontal section. The vertical section was completed with casing andgravel pack tubing was used in the horizontal section. Packers and flowcontrol valves were used to isolate the horizontal section into threezones. A FO-DTS was deployed along the entire length of the borehole.This example used a single-ended configuration for the FO-DTS although aloop configuration could have been used. An electrical quartz pressureand temperature gauge was provided in each of the three horizontalzones. In this example, the electrical quartz pressure and temperaturegauges were associated with the flow control devices, although suchassociation is not a limitation. Other gauge configurations andplacements are contemplated within the scope of the present invention.

[0054]FIG. 6 shows the temperature measured by the DTS in this exampleversus the distance into the borehole from the surface. The horizontalsection can be roughly correlated with portion of FIG. 6 where thetemperature profile is approximately flat, from approximately 1050meters to total depth. In this section of the temperature profile, threetemperature spikes can be seen which correlate to three splices in theFO-DTS. While the location of splices along the length of a sensor canbe noted during deployment for later consideration, these splicesnevertheless contribute to loss of calibration of the distributed sensorin the localized areas near the splices. Similar localized areas ofcalibration loss may occur near bends, strains or other deleteriousinfluences affecting the optical fiber distributed sensor.

[0055]FIG. 7 shows a relatively constant offset of approximately 6° C.throughout the entire DTS profile as compared to a baseline temperaturereference as measured in controlled conditions or provided asmanufacturer's information. This represents a substantial variation asunder ideal conditions a FO-DTS can provide accuracy withinapproximately 1° C. Use of independent temperature sensors permittedcorrections for this offset. The temperature was measured using anindependent electrical temperature gauge near 1000 meters with aprecision of 0.1 degree C. or less and a correction calculated as thedifference between the temperature measurement of the independent gaugeand the temperature measured by the FO-DTS. The entire DTS profile wasthen offset with this correction and also set to the correct temperatureas determined by the downhole temperature gauge.

[0056]FIG. 8 shows the offset of DTS measurements taken at two differenttimes for the portion of the DTS in the borehole. Three electricalquartz pressure and temperature gauges were used to measure temperaturein the borehole at locations near the splices in the FO-DTS, asindicated by the spikes in the DTS temperature measurements in FIG. 6.These gauges had an accuracy of approximately 0.1° C., therebypermitting correction of the FO-DTS measurements to an level within theaccuracy of FO-DTS systems.

[0057] An embodiment of the present invention contemplates usingtemperature measurements from independent temperature gauges both tocorrect for gross temperature errors from physical traumas such assplices and optical fiber loss in FIG. 6 and to increase the accuracy ofthe DTS measurement, across the entire fiber length, to the level of theindependent reference gauge, that is to a level substantially betterthan 1 degree C. The present invention applies to both single ended andloop DTS configurations.

[0058] Another embodiment of the present invention is a method ofobtaining corrected temperature measurements along a FO-DTS deployed ina loop configuration, comprising use of measurements from both ends ofthe optical fiber. This method comprises estimating the cumulative erroralong an optical fiber utilizing measurements taken from back-scatteredsignal (light) from both ends of the fiber loop.

[0059] In general along FO distributed sensors, the backscattered energyE₁ at a point i is: $\begin{matrix}{E_{i} = {E_{input}F_{i}{\exp ( {\sum\limits_{n = 0}^{i}R} )}}} & (3)\end{matrix}$

[0060] where E_(input) is the input power, F_(i) is the coefficient ofbackscattering at point i and is a known function of temperature forvarious optical fibers, and $\sum\limits_{n = 0}^{i}R$

[0061] is the additive error along the distributed fiber from the inputpoint to i. The input power, E_(input), can be measured. Also,backscattered signal at particular wavelengths can be measured at thepoints along the FO distributed sensor. Alternatively, the ratio ofparticular wavelengths, such as the SAR, can be measured at points alongthe FO distributed sensor. Typically this measurement point is theenergy input point. After energy E_(input) is input, backscatteredenergy E_(i) occurs and signals from the backscattered energy can bemeasured; the backscattered signal measured at the input point isdesignated as S_(i)=ln (E_(i)), for any point i. This measuredbackscattered signal from point i includes the signal S_(i) at point iand the additive error or loss of signal along the distributed fiberfrom the measurement point to i. This embodiment of the inventioncomprises a method to estimating the additive error along thedistributed fiber to point i and calculating the true signal S_(i) atpoint i using this estimation and the measured backscattered signal.

[0062] The relationship between backscattered signal S and input energyis known as:

S _(i)=ln (E _(input) F _(i))  (4)

[0063] where S_(i) is the backscattered energy at i and F_(i) is thecoefficient of backscattering at point i. Using a method of the presentinvention to determine S_(i), and measuring E_(input), F_(i) can becalculated. Then F_(i) can be used with known manufacture or baselinereference functions for various types and configurations of opticalfiber to determine the temperature at point i.

[0064] One method of the present invention comprises providing a pulseof optical energy at one end of the loop, end Y, and recordingbackscattered wavelengths at points i along the optical fiber. As anexample, this methodology is explained in terms of measuring the Stokesand anti-Stokes wavelengths in from each end of the loop configurationoptical fiber. It is expressly contemplated within the scope of thepresent invention that this methodology applies to other types ofmeasurements that may demonstrate cumulative error such as temperaturemeasurements, measurements of other wavelengths, or measurements ofwavelength-related variables such as SAR. It is also contemplated thatthe present invention may be used with measurements of the Stokeswavelength only, measurement of the anti-Stokes wavelength only, or theSAR only.

[0065] The Stokes signal contribution is designated at S and theanti-Stokes signal contribution is designated as A. This methodcomprises providing optical energy to one end, end Y, of a loop opticalfiber sensor. The backscattered signals received at end Y are recorded,and the intensity of the signals or the loss variation between one pointi and the next point i+1 is used to populate two arrays: Sy for thesignal at the Stokes wavelength and Ay for the signal at the anti-Stokeswavelength. Then optical energy is provided at the other end of theoptical fiber loop, end Z, and recording backscattered wavelengths atpoints along the optical fiber. The backscattered signals received atend Z are recorded, and the intensity of the signals or the lossvariation between one point i and the next point i+1 is used to populatetwo arrays: Sz for the signal at the Stokes wavelength and Az for thesignal at the anti-Stokes wavelength.

[0066] The loss variation between one point, i, and the next point, i+1,are represented by an array E_(S) and an array E_(A) for Stokes andanti-Stokes signals, wherein E represents the difference in intensity orthe loss between points i and i+1.

[0067] For arrays of k points, $\begin{matrix}{{Sy}_{i} = {S_{i} + {\sum\limits_{j = 0}^{i}{E_{S}}_{j}}}} & (5) \\{{Sz}_{i} = {S_{i} + {\sum\limits_{j = {i + 1}}^{k}{E_{S}}_{j}}}} & (6) \\{{A\quad y_{i}} = {{A\quad S_{i}} + {\sum\limits_{j = 0}^{i}{E_{A}}_{j}}}} & (7) \\{{A\quad z_{i}} = {{A\quad S_{i}} + {\sum\limits_{j = {i + 1}}^{k}{E_{A}}_{j}}}} & (8)\end{matrix}$

[0068] At a given point i along the optical fiber, assuming equivalentoptical energy input at ends Y and Z, the difference between the Stokesmeasurement determined from end Y and the Stokes measurement determinedfrom end Z is: $\begin{matrix}{{Ds}_{i} = {{{Sy}_{i} - {Sz}_{i}} = {{\sum\limits_{j = 0}^{i}{E_{S}}_{j}} - {\sum\limits_{j = {i + 1}}^{k}{E_{S}}_{j}}}}} & (9)\end{matrix}$

[0069] and the difference between the anti-Stokes measurement determinedfrom end Y and the anti-Stokes measurement determined from end Z is$\begin{matrix}{{Da}_{i} = {{{A\quad y_{i}} - {A\quad z_{i}}} = {{\sum\limits_{j = 0}^{i}{E_{A}}_{j}} - {\sum\limits_{j = {i + 1}}^{k}{E_{A}}_{j}}}}} & (10)\end{matrix}$

[0070] The increment of each element gives the double of the losscontribution for each point: $\begin{matrix}{{{{\Delta \quad s_{i}} = {{{Ds}_{i} - {Ds}_{i - 1}} = {{{\sum\limits_{j = 0}^{i}{E_{S}}_{j}} - {\sum\limits_{j = {i + 1}}^{k}{E_{S}}_{j}} - {\sum\limits_{j = 0}^{i - 1}{E_{S}}_{j}} + {\sum\limits_{j = i}^{k}{E_{S}}_{j}}} = {2{E_{S}}_{j}}}}}{and}}\quad} & (11) \\{{\Delta \quad a_{i}} = {{{Da}_{i} - {Da}_{i - 1}} = {{{\sum\limits_{j = 0}^{i}{E_{A}}_{j}} - {\sum\limits_{j = {i + 1}}^{k}{E_{A}}_{j}} - {\underset{j = 0}{\overset{i - 1}{\quad\sum}}\quad {E_{A}}_{j}} + {\sum\limits_{j = i}^{k}{E_{A}}_{j}}} = {2{E_{A}}_{j}}}}} & (12)\end{matrix}$

[0071] This enables the estimation of the signal contribution due totemperature only as: $\begin{matrix}{{S_{i} = {{Sy}_{i} - {\sum\limits_{j = 0}^{i}\frac{\Delta \quad s_{j}}{2}}}}{and}} & (13) \\{A_{i} = {{Ay}_{i} - {\sum\limits_{j = 0}^{i}\frac{\Delta \quad a_{j}}{2}}}} & (14)\end{matrix}$

[0072] With the loss contribution to the signal extracted from bothStokes and anti-Stokes signals, the SAR can be calculated and thetemperature at each point can be estimated using Equation 1 withoutneeding to know the differential loss factor for the two wavelengths.The same method could be used to correct for the influence of loss fordata already processed to obtain the temperature information, providedthe error is cumulative.

[0073] Similarly, the present invention contemplates direct measurementof the SAR in the forward and reverse directions and estimation of thesignal loss contribution owing to temperature as follows:

[0074] For arrays of k points, $\begin{matrix}{{SARy}_{i} = {{SAR}_{i} + {\sum\limits_{j = 0}^{i}E_{{SAR}_{j}}}}} & (15) \\{{SARz}_{i} = {{SAR}_{i} + {\sum\limits_{j = {i + 1}}^{k}E_{{SAR}_{j}}}}} & (16)\end{matrix}$

[0075] The increment of each element gives the double of the losscontribution for each point: $\begin{matrix}{{\Delta \quad {SAR}_{i}} = {{{DSAR}_{i} - {{DSAR}_{i - 1}{\sum\limits_{j = 0}^{i}E_{{SAR}_{j}}}} - {\sum\limits_{j = {i + 1}}^{k}E_{{SAR}_{j}}} - {\sum\limits_{j = 0}^{i - 1}E_{{SAR}_{j}}} + {\sum\limits_{j = i}^{k}E_{{SAR}_{j}}}} = {2E_{{SAR}_{j}}}}} & (17)\end{matrix}$

[0076] This enables the estimation of the signal contribution due totemperature only as: $\begin{matrix}{\quad {{SAR}_{i} = {{SARy}_{i} - {\sum\limits_{j = 0}^{i}\quad \frac{\Delta \quad {SAR}_{j}}{2}}}}} & (18)\end{matrix}$

[0077]FIG. 9 shows an example of data collected from energy input at oneend of an optical fiber (referred to as the forward direction) andenergy input at the opposite end of the optical fiber (referred to asthe reverse direction) and the corrected data using the present method.In this case, the fiber passes through cold water at approximately 305 malong its length and the fiber passes through a hot oven atapproximately 350 m along its length.

[0078] Localized regions of high loss were induced by coiling the fiberaround a pencil at approximately 325 m and approximately 360 m along thefiber length, just before and after the oven. The forward and backwarddata do not coincide due to the difference in attenuation for the Stokesand anti-Stokes data, mainly at the high loss points. The data werecorrected even without the use of the Stokes and anti-Stokes raw data.

[0079] It is contemplated within the scope of this invention that theembodiments of the invention are combinable in complementaryconfigurations. For example, in conjunction with providing light atStokes and anti-Stokes wavelengths, measuring the backscattered signals,populating arrays and calculating errors, an independent temperaturemeasurement could be provided to calibrate the calculated error at theindependent temperature measurement location. The present inventioncontemplates measuring temperature or calibrating borehole properties byproviding such multiple embodiments simultaneously or at different timesin a borehole. The present invention further contemplates use of FO-DTScomprising SMF, MMF, or a combination of SMF and MMF.

[0080] There have been described and illustrated herein severalembodiments of methods and apparatus for measuring differentialtemperature with fiber optic sensors distributed. While particularembodiments of the invention have been described, it is not intendedthat the invention be limited thereto, as it is intended that theinvention be as broad in scope as the art will allow and that thespecification be read likewise. It will therefore be appreciated bythose skilled in the art that yet other modifications could be made tothe provided invention without deviating from its spirit and scope as soclaimed.

1. A method of determining temperature using a fiber optic distributedtemperature sensor, comprising: a) providing optical energy at theStokes wavelength to a fiber optic distributed temperature sensor; b)receiving backscattered signal at Stokes wavelength and measuring itsintensity; c) providing optical energy at the anti-Stokes wavelength tosaid fiber optic distributed temperature sensor; d) receivingbackscattering signal at anti-Stokes wavelength and measuring itsintensity; e) calculating the attenuation ratio between thebackscattered signal at Stokes and anti-Stokes wavelengths at selectedpositions along the optical fiber; f) repeating steps a through e at adifferent time; g) calculating the change in the attenuation ratio withtime of the Stokes and anti-Stokes wavelengths; h) calculating acorrected SAR by multiplying the measured SAR and said change inattenuation ratio with time; and h) using the corrected SAR to adjusttemperatures measured by said fiber optic distributed temperaturesensor.
 2. The method as claimed in claim 1, wherein a tunable lightsource provides said optical energy at Stokes wavelength and saidoptical energy at anti-Stokes wavelength.
 3. The method as claimed inclaim 1, wherein said fiber optic distributed temperature sensor isdeployed in a borehole.
 4. A method of calibrating fiber opticdistributed temperature sensor measurements comprising: a) providingoptical energy at the Stokes wavelength to a fiber optic distributedtemperature sensor; b) receiving backscattered signal at Stokeswavelength and measuring its intensity; c) providing optical energy atthe anti-Stokes wavelength to said fiber optic distributed temperaturesensor; d) receiving backscattering signal at anti-Stokes wavelength andmeasuring its intensity; e) calculating the attenuation ratio betweenthe backscattered signal at Stokes and anti-Stokes wavelengths atselected positions along the optical fiber; f) repeating steps a throughe at a different time; g) calculating the change in the attenuationratio with time of the Stokes and anti-Stokes wavelengths; h) measuringtemperature using the fiber optic distributed temperature sensor; and i)calculating a corrected SAR by multiplying the measured SAR and saidchange in attenuation ratio with time.
 5. The method as claimed in claim4, wherein a tunable light source provides said optical energy at Stokeswavelength and said optical energy at anti-Stokes wavelength.
 6. Themethod as claimed in claim 4, wherein said fiber optic distributedtemperature sensor is deployed in a borehole.
 7. A method of determiningtemperature along a fiber optic distributed temperature sensor,comprising the steps of: a) measuring temperature using a fiber opticdistributed temperature sensor; b) measuring temperature using anindependent temperature sensor located along said fiber opticdistributed temperature sensor; c) calculating the difference intemperature ΔT₁ between the temperature measured by said independenttemperature sensor and the temperature measured by said fiber opticdistributed temperature sensor at the location of said independenttemperature sensor; and d) using ΔT₁ to adjust temperatures measured bythe fiber optic distributed temperature sensor.
 8. The method as claimedin claim 7, wherein said fiber optic distributed temperature sensor isdeployed in a borehole.
 9. The method as claimed in claim 7, whereinsaid fiber optic distributed temperature sensor further comprises afiber Bragg grating.
 10. A method of calibrating fiber optic distributedsensor measurements, comprising the steps of: a) measuring a parameterof interest along a fiber optic distributed sensor; b) measuring saidparameter of interest by an independent sensor; c) determining thedifference ΔT₁ between said parameter of interest measured by saidindependent sensor and said parameter of interest measured by said fiberoptic distributed sensor at the location of said independent sensor; d)using ΔT₁ to adjust said parameter of interest as measured by saidoptical fiber distributed sensor.
 11. The method as claimed in claim 10,wherein said fiber optic distributed sensor is deployed in a borehole.12. The method as claimed in claim 10, wherein said fiber opticdistributed sensor further comprises a fiber Bragg grating.
 13. Themethod as claimed in claim 10, wherein said parameter of interest istemperature.
 14. The method as claimed in claim 10, wherein saidparameter of interest is pressure.
 15. The method as claimed in claim 11wherein the independent sensor is placed at a location of particularinterest in said borehole.
 16. A calibrated fiber optic distributedtemperature sensor deployed in a borehole, comprising a fiber opticdistributed temperature sensor comprising at least one fiber Bragggrating and a least one independent temperature sensor deployed in saidborehole.
 17. A method for accurately determining a borehole parametercomprising: a) deploying a fiber optic distributed sensor in a borehole;b) providing an independent sensor in said borehole along said fiberoptic distributed sensor; c) measuring a borehole parameter using saidfiber optic distributed sensor; d) measuring said borehole parameterusing said independent sensor, wherein said independent sensor providesa more accurate measurement than said fiber optic distributed sensor; e)calculating the difference between the borehole parameter measurementmade by the independent sensor and the borehole parameter measurementmade using the fiber optic distributed sensor at the location of theindependent sensor; and f) adjusting the borehole parameter measurementsmade using the fiber optic distributed sensor by said difference. 18.The method as claimed in claim 17, wherein said borehole parameter istemperature.
 19. The method as claimed in claim 17, wherein saidborehole parameter is pressure.
 20. The method as claimed in claim 17,wherein said fiber optic distributed sensor further comprises FBG.
 21. Amethod of determining a parameter of interest along an optical fiberdistributed sensor, comprising the steps of: a) providing optical energyto a fiber optic distributed sensor; b) measuring a parameter ofinterest using said fiber optic distributed sensor; c) measuring saidparameter of interest using at least one fiber Bragg grating in theoptical fiber distributed sensor; d) determining the difference in saidparameter of interest ΔT_(2j) between the parameter of interestmeasurement by the at least one fiber Bragg grating and the parameter ofinterest measured using said fiber optic distributed sensor at thelocation of the at least one fiber Bragg grating reference, wherein j isthe number of fiber Bragg gratings; and e) using ΔT_(2j) to adjustparameter of interest measurements made using the optical fiberdistributed sensor.
 22. The method as claimed in claim 21, wherein saidfiber optic distributed sensor is deployed in a borehole.
 23. The methodas claimed in claim 21, wherein said borehole parameter is temperature.24. The method as claimed in claim 21, wherein said borehole parameteris pressure.
 25. The method as claimed in claim 21, wherein said fiberoptic distributed sensor is a single mode fiber.
 26. The method asclaimed in claim 21, wherein said fiber optic distributed sensor is amultimode fiber.
 27. The method as claimed in claim 21, wherein saidfiber optic distributed sensor is a combination of single mode andmultimode fibers.
 28. A method of determining temperature along anoptical fiber distributed temperature sensor, comprising the steps of:a) providing optical energy to a fiber optic distributed temperaturesensor; b) measuring temperature using said fiber optic distributedtemperature sensor; c) measuring temperature using at least oneindependent temperature sensor located along said fiber opticdistributed temperature sensor; d) measuring temperature using at leastone fiber Bragg grating temperature in the optical fiber distributedtemperature sensor, e) determining the difference in temperature ΔT_(1i)between said at least one independent temperature sensor and thetemperature measured by said fiber optic distributed temperature sensorat the location of said at least one independent temperature sensor,wherein i is the number of independent temperature sensors; f)determining the difference in temperature ΔT_(2j) between thetemperature measured by said at least one fiber Bragg grating and thetemperature measured by said distributed temperature sensor at thelocation of said at least one fiber Bragg grating reference, wherein jis the number of fiber Bragg gratings, and g) using ΔT_(1i) and ΔT_(2j)to adjust temperatures measured by said optical fiber distributedtemperature sensor.
 29. The method as claimed in claim 28, whereinadjustments to temperatures measured by the optic fiber distributedsensor are made between the locations of the i independent temperaturesensors and the location of the j fiber Bragg gratings.
 30. The methodas claimed in claim 28, wherein said fiber optic distributed temperaturesensor is deployed in a borehole.
 31. The method as claimed in claim 28,wherein said adjustment to temperatures measured by the optic fiberdistributed sensor varies spatially along the fiber optic distributedtemperature sensor.
 32. The method as claimed in claim 31, wherein theadjustment is spatially varied based on the location of known featuresof the fiber optic distributed temperature sensor.
 33. A method ofcalibrating optical fiber distributed temperature sensor measurements,comprising the steps of: a) providing optical energy to a fiber opticdistributed temperature sensor; b) measuring temperature using at leastone independent temperature sensor located along said fiber opticdistributed temperature sensor; c) measuring temperature using at leastone fiber Bragg grating in said optical fiber distributed temperaturesensor; and d) using the temperature measured by the at least oneindependent temperature sensor and the temperature measured by the atleast one fiber Bragg grating to adjust the temperature measured by saidfiber optic distributed temperature sensor.
 34. The method as claimed inclaim 33, wherein said fiber optic distributed sensor is deployed in aborehole.
 35. A method of determining temperature along a fiber opticdistributed temperature sensor, comprising the steps of: a) providingoptical energy into one end of a fiber optic distributed temperaturesensor and transmitting in the forward direction; b) measuring thebackscattered signal at Stokes wavelength; c) determining theincremental loss in signal at Stokes wavelength at various points alongthe sensor from the optical energy input in the forward direction andpopulating an array; d) measuring the backscattered signal atanti-Stokes wavelength; e) determining the incremental loss in signal atanti-Stokes wavelength at various points along the sensor from theoptical energy input in the forward direction and populating an array;f) providing optical energy into the other end of said fiber opticdistributed temperature sensor and transmitting in the reversedirection; g) measuring the backscattered signal at Stokes wavelength;h) determining the incremental loss in signal at Stokes wavelength atvarious points along said sensor from the optical energy input in thereverse direction and populating an array; i) measuring thebackscattered signal at anti-Stokes wavelength; j) determining theincremental loss in signal at anti-Stokes wavelength at various pointsalong said sensor from the optical energy input in the reverse directionand populating an array; k) determining the difference between themeasurement incremental loss variation in Stokes wavelength in theforward and the reverse directions for any given point along saidsensor; l) determining the difference between the measurementincremental loss variation in anti-Stokes wavelength in the forward andthe reverse directions for any given point along said sensor; m)correcting the measured Stokes wavelength at i by${S_{i} = {{Sf}_{i} - {\sum\limits_{j = 0}^{i}\quad \frac{\Delta \quad s_{j}}{2}}}};$

n) correcting the measured anti-Stokes wavelength at i by${A_{i} = {{Af}_{i} - {\sum\limits_{j = 0}^{i}\quad \frac{{\Delta a}_{j}}{2}}}};$

o) measuring temperature using said fiber optic distributed temperaturesensor; and p) adjusting measured temperature using corrected StokesS_(i) and anti-Stokes A_(i) wavelengths.
 36. The method as claimed inclaim 35, wherein a tunable light source provides said optical energy atStokes wavelength and said optical energy at anti-Stokes wavelength. 37.The method as claimed in claim 35, wherein said fiber optic distributedtemperature sensor is deployed in a borehole.
 38. The method as claimedin claim 35, wherein said fiber optic distributed temperature sensorfurther comprises a fiber Bragg grating.
 39. A method of calibratingfiber optic distributed temperature sensor measurements, comprising thesteps of: a) providing optical energy into one end of a fiber opticdistributed temperature sensor; b) receiving backscattered signals atStokes and anti-Stokes wavelengths at said one end and populating anarray of signals at Stokes wavelengths and an array of signals atanti-Stokes wavelengths; c) providing optical light into the oppositeend of said fiber optic distributed temperature sensor; d) receivingbackscattered signals at Stokes wavelength and at anti-Stokes wavelengthat said opposite end and populating an array of signals at Stokeswavelengths and an array of signals at anti-Stokes wavelengths; e)correcting the measured signals received at Stokes wavelengthbackscattered at i by${S_{i} = {{Sf}_{i} - {\sum\limits_{j = 0}^{i}\quad \frac{\Delta \quad s_{j}}{2}}}};$

and f) correcting the measured signals received at anti-Stokeswavelength backscattered at i by$A_{i} = {{Af}_{i} - {\sum\limits_{j = 0}^{i}\quad {\frac{{\Delta a}_{j}}{2}.}}}$


40. A method of measuring a parameter of interest along a fiber opticdistributed sensor, comprising the steps of: a) providing optical energyinto one end of a fiber optic distributed sensor and transmitting in theforward direction; b) measuring the backscattered signal; c) determiningthe incremental loss in signal from the optical energy input in theforward direction at various points along said sensor and populating anarray; d) providing optical energy into the other end of said fiberoptic distributed sensor and transmitting in the reverse direction; e)measuring the backscattered signal; f) determining the incremental lossin signal from the optical energy input in the reverse direction atvarious points along the sensor and populating an array; g) determiningthe average of the incremental loss in signal in the forward and theincremental loss in signal in the reverse directions for any given pointalong the fiber optic distributed sensor; and h) adjusting parametermeasurements made using the fiber optic distributed sensor by saidaverage.
 41. The method as claimed in claim 40, wherein thebackscattered signal is measured at the Stokes wavelength.
 42. Themethod as claimed in claim 40, wherein the backscattered signal ismeasured at the anti-Stokes wavelength.
 43. The method as claimed inclaim 40, wherein the backscattered signal is measured at the Stokeswavelength and the anti-Stokes wavelength.
 44. The method as claimed inclaim 40, wherein the backscattered signal is measured as the SAR. 45.The method as claimed in claim 40, wherein a tunable light provides theoptical energy.
 46. The method as claimed in claim 40, wherein saidfiber optic distributed sensor is deployed in a borehole.
 47. The methodas claimed in claim 40, wherein said fiber optic distributed sensorfurther comprises a fiber Bragg grating.
 48. The method as claimed inclaim 40, wherein said fiber optic distributed sensor further comprisesan independent sensor.